CMake bot script

 - Add CMake v3.3.1 (latest) to DEPS.
 - Add cmake/bot-cmake.sh to bootstrap CMake then build Skia using that.

Works on my Mac and Linux box, both with no system CMake installation.

CMake will be ~100M on disk.  The first bootstrap takes a couple minutes,
and a no-op re-run of bot-cmake.sh takes 15-20 seconds.  I thought about
having bot-cmake.sh fetch CMake instead of DEPS, but I'm not sure I can
handle updates, etc. as robustly as it can.

This will only work on Linux and Mac.  CMake requires an older CMake on
Windows.  It doesn't have an equivalent ./bootstrap there.  Will have to
think about how Windows bots will work!

BUG=skia:4269

Review URL: https://codereview.chromium.org/1339603003
diff --git a/DEPS b/DEPS
index 28251d0..c21f9f5 100644
--- a/DEPS
+++ b/DEPS
@@ -9,6 +9,7 @@
   #   - can use use our existing t_p/e/libjpeg instead of pulling it for Android?
 
   "third_party/externals/angle2"  : "https://chromium.googlesource.com/angle/angle.git@f9ac8fea307532e31cba6240801b4a56483591f8",
+  "third_party/externals/cmake"   : "https://cmake.googlesource.com/cmake.git@v3.3.1",
   "third_party/externals/freetype": "https://skia.googlesource.com/third_party/freetype2.git@VER-2-5-0-1",
   "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@08429da7955a98ef764fafa223dd7de73f654b2d",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@0.9.35",